despawned chickens

Hello everyone Im here to file a bug/complaint about my chickens randomly despawning. I just recently made a hopper system to collect the eggs as they are laid, I used about 7 spawn eggs to start and then I left the server to play another game but when I came back... (i hope the picture shows)on each of these hoppers there was a baby chick but now they are gone and my /entc has gone down there not anywhere on my res or outside my res. I'm not asking for a refund (but a refund would be nice) but I'm asking for a fix (also when I realized this I typed in chat about it and my neighbor FUCreeper said his animals despawn occasionally too) this has never happened to me before is this a know bug that's being worked on or is this fairly new?(also im still a bit of a noob so im not sure if Ive said everything or even posted in the right spot if I have would a staff member move/fix this?this happened on SMP3